The present invention is directed to a high pressure control valve and in particular to a new and improved soft seat design.
One of the principal functions of a high pressure control valve is to provide a complete and leak-proof shut off of fluid flow when the valve is in the closed position. Typically, this is accomplished by mating an axially movable valve plug with a stationary valve seat, whereby the valve plug and seat isolate the high pressure fluid from the fluid outlet portions of the control valve. In most conventional valve designs, the valve seat is made from metal to provide a so-called "hard-seat" arrangement. The valve plug is also made from metal and therefore the fluid flow shut off is achieved by a metal-to-metal contact between the valve plug and the valve seat. In such a hard-seat arrangement, it is critical that the valve plug and seat precisely mate with one another to completely shut off and isolate the high pressure fluid from the valve outlet. The manufacture of each of the valve plug and seat from metal is advantageous in a high pressure application in that the metal components are fairly resistant to the deleterious effects of the high pressure fluid flow including cavitation and other eroding effects caused by the high pressure fluid flow and the generally high pressure drops experienced by the fluid flow as it passes through the valve structure. Nevertheless, it is widely recognized in the valve industry that it is virtually impossible to obtain a zero-leakage mating between the valve plug and seat when utilizing a hard metal-to-metal contact arrangement.
Moreover, in time, the high pressure fluid flow through the valve causes wear and tear on the metal components. In addition, inpurities found within the fluid flow may become trapped between the valve plug and valve seat during valve shut off to thereby pit and otherwise damage the valve plug and/or valve seat. It should be understood that the normal wear and tear and possible damage to the valve components caused by impurities found within the fluid flow will not occur uniformly throughout the mating portions of the metal valve plug and metal seat. Thus, in time, the mating relationship between the valve plug and seat will become less precise resulting in a greater amount of fluid leakage when the valve is in the shut off position. Accordingly, the valve will gradually become less effective as a means to completely interrupt fluid flow. In certain severe duty valve applications, such as pump recirculation, complete fluid isolation is critical and the gradual and uneven erosion of the metal components of the valve make such hard seat designs somewhat less than desirable in the critical, severe duty applications.
In order to overcome the shortcomings of the hard-seat design, those skilled in the art have proposed the use of a so-called "soft seat" design. In such a valve arrangement, the seat is made from a resilient, deformable material, whereby the contact load placed upon the valve plug will cause the valve plug to become imbedded within the soft seat and thereby conform the soft, deformable seat precisely to the contour and configuration of the valve plug mating portions. In this manner, the soft seat provides a precise mating each and every time the plug is mated with the seat. Moreover, the high degree of seat conformance achieved with a soft seat design is obtainable with stem forces that are far lower than the stem forces required to achieve a good leak-tight shut off in a metal-to-metal arrangement. Accordingly, a zero leakage valve shut off may be obtained far more readily and easily in a soft seat design than in a hard seat design. However, a problem associated with the soft seat arrangement is the rather rapid erosion of the resilient deformable material utilized to manufacture the soft seat. Typically, as the valve plug is lifted from the valve seat, a very high pressure fluid flow is immediately throttled directly over the valve seat as fluid flow is permitted to resume between the valve plug and the valve seat. It is this throttling effect which generally erodes the soft seat valve material. As a consequence of the relatively rapid wear down of the soft valve seat, such soft seat valves require a more than desirable amount of down time in order to rebuild or replace the soft seat. Indeed, the rapid erosion may preclude the use of a soft seat in certain high pressure applications.
It is a primary objective of the present invention to provide a soft seat valve with additional novel means to displace the deleterious throttling effects of the fluid flow away from the soft valve seat. Generally, the invention comprises a high pressure control valve including a soft seat made from a resilient, deformable material. In accordance with a significant feature of the invention, the control valve is provided with a main valve plug which is matable with the soft seat and an additional, auxiliary throttle plug which is coaxial with and axially movable relative to the main valve plug. The throttle plug is arranged to mate with a second metal valve seat which is concentric with and arranged in an area removed from the soft valve seat. In accordance with the invention, the axial movements of each of the main valve plug and the throttle plug are coordinated in a manner whereby the primary throttling areas for the fluid flow are displaced to portions of the valve structure which are remote from the soft seat, as will appear.
As contemplated by the preferred embodiment of the invention, the main plug comprises a generally cylindrical, at least partially hollow plug having a valve seat-engaging rim which circumscribes the at least partially hollow portion of the plug. The throttle plug is axially received within the hollow portion of the main plug and is associated with a throttle plug retaining means arranged to retain the throttle plug within the main plug hollow portion while accommodating a limited, axial movement of the throttle plug relative to the main plug. Moreover a biasing means urges the throttle plug away from the main plug whereby one end of the throttle plug ordinarily protrudes from the hollow portion of the main plug beyond the valve seat-engaging rim. When in the closed position, the rim of the main plug contacts the soft seat under pressure from a valve stem to form a leak-tight, fully conformed mating between the rim and the deformable, resilient seat. In this manner, a highly effective fluid flow shut off is achieved. In addition, the protruding end of the throttle plug will be in a mating relation with the second, metal valve seat.
Pursuant to the invention, the valve is operated such that during the initial opening of the valve, the rim of the main plug is displaced from the soft seat while unbalanced pressure effects, as will be more fully described below, and the biasing means act to retain the throttle plug in contact with the second, metal seat. At this point, the only fluid flow will be leakage through the metal-to-metal contact of the throttle plug and seat and around various piston rings arranged between the main plug and the throttle plug. However, as should be understood, the leakage is primarily throttled across either the metal seat or the piston rings and, to advantage, these components may be made from erosion resistant, stainless steel.
As the valve continues to open, the main plug will continue to lift from the soft seat. Eventually, the retaining means will cause the throttle plug to move in tandem with the main plug to thereby displace the throttle plug away from the second, metal seat. At this point, the fluid will begin to flow freely through the valve and the primary fluid throttling area will be across the second, metal seat. Inasmuch as the second, metal seat is spaced from the soft seat, the novel main plug-throttle plug arrangement of the invention provides the advantages of a soft seat valve shut off in a manner whereby the primary throttling of the fluid flow is caused to occur at areas of the valve structure which are remote from the soft seat.
In accordance with another feature of the invention, the metal seat is arranged upstream from the soft seat and a fluid flow restrictor means is arranged downstream from the soft seat. Accordingly, the fluid flow restrictor means will serve as an additional principal throttling area to further reduce erosive fluid forces at the soft seat. Moreover, when the valve is in the fully opened position, all of the throttling will be across the fluid flow restrictor means.
Thus, the present invention provides a control valve with the highly advantageous zero leakage shut off obtainable by utilizing a soft seat while, at the same time, greatly reducing the erosive effects of the fluid flow upon the soft seat by displacing the principal throttling areas for the fluid flow away from the soft seat. The rim of the main plug will, in each valve closing operation, fully conform the deformable, resilient seat to its exact controu and configuration. The throttle plug will operate, in both valve opening and valve closing operations, to provide a primary throttling area for the fluid flow comprising an erosion resistant surface that is spaced from the soft seat. The invention, therefore, teaches an effective and straight forward means for shielding the soft seat from harmful fluid flow effects within a valve structure arranged to shut off high pressure fluid flow by the exact mating between the main plug and the soft seat.